Question

I’ve got the following bit of code (that another developer wrote) that I need to modify because the ExecuteProc procedure fails. (The reason why it fails isn’t in the scope of this question. It’s a long story) but in scope is that I’m not sure why a NullReferenceException would occur. It’s in VB.NET (I’m not a VB.NET guy myself, so I’m not sure if this is something that is unique to VB.NET):

Imports System.Data Imports System.Data.SqlClient Imports System.Data.SqlTypes Imports System.Collections '(...) Public Overloads Function ExecuteProc( ByVal pvsProcName as String) As DataSet Dim oCommand As SqlCommand Dim oDataAdaptor As SqlDataAdapter = New SqlDataAdapter 'Other Stuff here    Try     oCommand = CommandGet(ConnectionGet)     'Do some more stuff that's irrelevant    Catch oException As Exception     Throw oException    Finally     ConnectionClose(oCommand.Connection)     oCommand.Dispose()     oDataAdaptor.Dispose()   End Try End Function 

I get the following warning:

Variable 'oCommand' is used before it has been assigned a value. A null reference exception could result at runtime.` 

Edit: I found the answer right before I posted the question. I posted it to keep it around (I hate putting ten minutes into something to dump it willy nilly).

My subsequent question for VB.NET folks is this:

What would be the difference in the following two intializations:

Dim oCommand As SqlCommand = Nothing 

And:

Dim oCommand as New SqlComand 

Answer 1

Well, the most obvious thing is that if CommandGet fails and throws an exception, oCommand won’t be set – it’ll still be null, so oCommand.Connection in the Finally block will throw a NullReferenceException.

In addition, I’d say:

• There’s no need for the Catch block
• Two Using blocks (one for the command, one for the adapter) would be better IMO than explicit Try/Finally blocks

EDIT: The difference between the two solutions is simple – one creates a ‘dummy’ command which you then ignore or dispose (but not both). The other explicitly sets a null reference, which you can check for.

I would use a third solution though – a Using block:

Using oCommand As CommandGet(ConnectionGet)   ' All the normal stuff in here ' End Using